Conventionally, as a smoothing method of smoothing an edge of a character intended for a color laser printer, for example, there has been provided a method of binarizing or ternarizing image data, determining whether the binarized or ternarized image data match a pattern in a predetermined pattern table, and, when determining that the binarized or ternarized image data match a pattern in the predetermined pattern table, replaces a notice pixel by a predetermined image signal (for example, refer to patent reference 1). Further, this patent reference 1 describes a method of carrying out edge determination on an input image, determining whether a notice pixel is placed either on a peripheral edge portion or on an image flat portion other than the peripheral edge portion, and switching whether or not to carry out a matching process according to the result of the determination, thereby providing a high-resolution output image.